Python for a Coding Assessment
A refresher path through the Python graph for someone who has written Python before and has an assessment coming: run and read, rebuild the core, then work the way an assessment expects.
Model data with classes and dataclasses
I can define a class that holds related data and the behaviour belonging to it, use a dataclass when the class is mostly data, and say when a dict or a function would have been the better choice.
- Domain
- Python Programming
- Level
- intermediate
- Status
- draft · v0.1.0
- Also called
- Python classes, Object-oriented Python, dataclass, __init__
- License
- CC BY-SA 4.0
Learn first
- Write functions with clear signatures foundation
- Choose the right built-in data structure foundation
Learn it
Scope
Giving a name to a thing your program keeps talking about, and putting the behaviour that belongs to it in the same place.
In scope: defining a class, __init__, instance attributes and methods, dataclass, and judging
when not to. Out of scope: inheritance hierarchies, properties, metaclasses and protocols.
When a class earns its place
A class is warranted when the same group of values keeps travelling together and the same operations keep being done to them. The signal is in the code you already have: three parallel lists indexed in step, a dict whose keys you must remember, a function taking six arguments that are really one thing.
It is not warranted when there is no behaviour. A bag of values that nothing acts on is a dict, or a dataclass if you want dotted access and a readable repr. A single operation with no state is a function. Most Python is functions on plain data, and that is a good thing.
What a dataclass gives you
@dataclass writes the boilerplate: __init__, a __repr__ that prints the field values, and
__eq__ comparing field by field rather than by identity. That last one is the reason it matters in
testing: two records with the same contents compare equal, so assert result == Expected(...) says
what you mean.
Declare fields with types; frozen=True makes instances immutable, which makes them hashable and
safe to share.
The method question
A method belongs on the class when it needs the instance’s own data. order.total() reads better
than total(order) and keeps the calculation next to the fields it uses. A function that merely
takes an instance and returns something unrelated is a function; putting it on the class only makes
the class longer.
Common misconceptions
- “Good Python is object-oriented.” Good Python is whatever makes the code plain. Classes are one tool; a great deal of excellent Python has none.
- “A dataclass is a struct.” It is a normal class with generated methods. You can add your own and it is still a dataclass.
- “
selfis passed magically.”selfis the instance, passed as the first argument by the attribute lookup. Knowing that makes theTypeErrorabout arguments obvious. - “Class attributes are per-instance defaults.” A mutable class attribute is shared by every instance, the same trap as a mutable default argument.
Notes for teachers
Start from code that hurts: three lists kept in step, or a dict whose shape is documented in a comment. Ask learners to name the thing, then introduce the class as the name becoming code.
Then ask for the opposite case — somewhere a class was written and a function would do — so the judgement runs both ways.
Resources
| Resource | Kind | Cost | Provider |
|---|---|---|---|
| The Python Tutorial: classes | reading | free | Python Software Foundation |
| dataclasses: data classes | reference | free | Python Software Foundation |
Courses that include it
Prove it
Evidence of mastery
- Replaces a group of parallel lists or a dict of dicts with a class or dataclass, and the code reads more clearly afterwards.
- Writes `__init__` and one method that operates on the instance's own data.
- Uses a dataclass and explains which methods it generated and what they do.
- Argues, for a given case, that a class is not warranted and a function or a dict is enough.
Assessments that measure it
No assessment yet.
Credentials that include it
No credential yet.
What learners show
Nothing to show yet. The publisher last published on 2026-09-20, and no aggregate for this outcome has reached the 50 learners a number needs. Open Degree does not track learners. About learner evidence.
What comes next
No outcome builds on this yet. Ask for the next concept.